1. Field of the Invention
The present invention generally relates to exhaust systems and more particularly relates to exhaust systems that are used with engines.
2. Description of the Related Art
Water vehicles often have an exhaust system that includes an exhaust gas passage for the gas discharged from an engine and a coolant passage for coolant that has been used to cool the engine. The exhaust gas passage and the coolant passage mix the exhaust gas and the coolant and then discharge the mixture outside of the vehicle.
Japanese Patent Publication No. Hei 7-156883 discloses water jet propelled boats equipped with this type of engine exhaust system. The exhaust system has an upstream section (on the engine side) that includes an inner pipe and an outer pipe. The inner pipe is positioned within the outer pipe. Exhaust gases pass inside the inner pipe, while coolant passes in a space defined between the inner pipe and the outer pipe. The length of the inner pipe is less than the length of the outer pipe. In a downstream section of the exhaust system, the exhaust gases and the coolant mix with each other and the mixture flows downstream and is ultimately discharged out of the water jet propelled boat. The inner pipe has a restriction having a reduced diameter. The restriction is formed at a downstream end of the inner pipe.
In the exhaust system, pulsations may occur when negative pressure waves propagate inside of the engine during its exhaust stroke. The restriction can suppress the reflection of such negative pressure pulses. Even if the pressure downstream of the restricted portion in the exhaust system becomes higher than that of the pressure upstream of the restriction, the restriction can suppress propagation of such negative pressure waves. The restriction can thus inhibit coolant from flowing upstream to the engine. Hence, the exhaust system can prevent water damage to the spark plugs and other parts of the engine, as well as ignition failure.
Unfortunately, the restriction described in the Japanese document may not sufficiently suppress the propagation of negative pressure waves. As such, negative pressure waves may often adversely affect engine performance and assist the movement of water upstream through the inner pipe and eventually to the engine.